Modern aircraft secondary power systems are becoming large with increasing demands for electric power and large power for starting the prime propulsion engines. These aircraft have a gas turbine powered auxiliary power unit (APU) on board to provide electrical, hydraulic and pneumatic power for operating systems aboard the aircraft when the propulsion engines are shut down as well as to provide power for starting the propulsion engines. Typically, pneumatic power, provided by a load compressor within the APU, has been used to start the propulsion engines. The load compressor has also been used to provide compressed air for the environmental control system (ECS) aboard the aircraft before and after the propulsion engines are started.
The main engine start (MES) requires much more compressed air from the APU than the ECS load. Since the load compressor is used for both the MES mode and the ECS mode, the APU is sized well above the needs by the MES requirement. The APU is thus oversized for the ECS mode and it is heavier, more costly and less efficient than if it were sized to match the ECS load alone. The heavier APU and its support structure detract from the aircraft fuel load or payload. The excess APU weight therefore shortens the range of the aircraft or reduces its payload.
Although increasing power demands that are due to factors other than MES requirements, such as more electric power cabin services, added avionics functions and large radar load for some military aircraft are also driving up the size and weight of APUs for modern aircraft, by far the most significant factor is the MES requirements.